Bonding in cyclophosphazenes

The nature of bonding in cyclophosphazenes can be imderstood by examining certain experimental facts and the arguments advanced to explain them. These can be summarized as follows [5,7, 9, 10]. 

It is easier to visualize the bonding in cyclophosphazenes by separating the a-component from the 7t-component that may be contributing together to the overall bonding picture in these compounds. These arguments can be summarized as follows. 

Faucher, J.-R, Labarre, J.-F., and Shaw, R. A., Bonding in cyclophosphazenes. Quantum and experimental support for Dewar s island model, Z. Naturforsch., 31b, 677 (1976). Mitchell, K. A. R., Noodleman, L., and Paddock, N. L., Xa scattered wave calculations of the ionization energies of N3P3F6 and N4P4F8, Chem. Phys. Lett., 47, 265 (1977). Noodleman, L., Westwood, N. P. C., and Mitchell, K. A. R., Ionization energies and electronic structure of the cyclic phosphazene as determined by UV photoelectron spectroscopy and Xa scattered wave methods. Chem. Phys. Lett., 58, 252 (1978). 

The structures of a large number of cyclophosphazene derivatives have been determined by X-ray diffraction, and the data obtained have helped to clarify the nature of the bonding in this class of compounds. Corbridge (125) has recently reviewed this work. 

As a consequence of this internal molecular stretching along the two-fold axis, atom P, appears to be in a pseudo-tetrahedral situation, the four associated phosphorus-nitrogen bonds being practically equal. Such a Td-like environment for a phosphorus atom belonging to a trimeric ring is unique in cyclophosphazenes and may explain the peculiar NMR behaviour manifested by P3 (see above). 

Finally, however, it must be mentioned that considerable theoretical work in main-group compounds suggests that the participation of d-orbitals in any type of bonding may be minimal. Alternative bonding models that avoid the use of d-orbitals and account for all the experimental facts in cyclophosphazenes may soon emerge from such studies. 

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